1. Field of the Invention
An article made of an alloy having a shape memory can be deformed at a low temperature from its original configuration. On application of heat, the article reverts back to its original configuration. Thus, the article "remembers" its original shape.
Nickel-titanium alloys, for example, are known to possess shape memory characteristics, i.e., the alloys undergo a reversible transformation from an austenitic state to a martensitic state with a change in temperature. The transformation is often referred to as a thermal elastic martensitic transformation. The reversible transformation of the alloy between the austenitic and the martensitic phase occurs over two different temperature ranges which are characteristic of the specific alloy. As the alloy cools it reaches a temperature (M.sub.s) at which the martensitic phase or R-phase starts to form, and finishes the transformation at a still lower temperature (M.sub.f). On reheating, it reaches a temperature (A.sub.s) at which austenite begins to reform and then a temperature (A.sub.f) at which the change back to austenite is complete. In the martensitic state, the alloy can be easily deformed When sufficient heat is applied to the deformed alloy, it reverts back to the austenitic state and returns to its original configuration.
2. Description of the Prior Art
Many alloys have been described in the art as possessing shape memory characteristics. Several are disclosed in the background section of commonly-owned U.S. patent application Ser. No. 07/609,377, filed Nov. 5, 1990 in the name of David AbuJudom, et al. and entitled "High Transformation Temperature Shape Memory Alloy". That description is incorporated herein by this specific reference and will not be repeated as the particular shape memory alloys useful in the present invention are not, in and of themselves, critical to the undertaking of or the scope of the present invention.
A most preferred shape memory alloy is disclosed in commonly-owned U.S. patent application Ser. No. 07/863,206, filed Apr. 3, 1992 in the name of AbuJudom, et al., a continuation-in-part application of the aforementioned application. In the latter, copper is added to SMA alloys including nickel, titanium and hafnium (with zirconium either added or as an impurity). In the background section of the latter application, additional patents are described which include copper as a component of nickel-titanium SMA materials. Other copper base SMA are also known, such as copper-aluminum-nickel and copper-zinc-aluminum alloys.
Many methods of forming shape memory alloys are also known. For example, Thoma, et al., U.S. Pat. No. 4,881,981, issued Nov. 21, 1989, relates to a method of producing shape memory alloys. The method includes the steps of increasing the internal stress level, forming the member to a desired configuration, and heat treating the member at a selected emory imparting temperature. Other processing methods are taught in the patents disclosed in the aforementioned AbuJudom, et al. applications.
It is also known from the applications and patents referred to above that shape memory alloys may be used to form tubes. See, for example, Harrison, U.S. Pat. No. 4,565,589 issued Jan. 21, 1986, which indicates that such materials have found use as pipe couplings. In Albrecht, et al., U.S. Pat. No. 4,412,872, issued Nov. 1, 1983, wires formed of shape memory alloys are processed into coil springs, as illustrated in FIG. 8 of that patent. Albrecht, et al., U.S. Pat. No. 4,518,444, issued May 21, 1985 discloses that certain shape memory alloys may be formed in the configuration of "bars, tubes, profiles, wires, sheets or bands", with the patent focusing on the particular materials to be used in such applications.
While shape memory alloys, in and of themselves, are relatively well known, this technology is still maturing and researchers are still investigating various ways of capitalizing on the unique properties of such materials. Problems still to be addressed include the development of viable techniques for forming a particular article, the selection of alloys having transformation temperatures at desired temperature ranges and techniques for actually heating or cooling the elements during use to cause such transformations to occur.
The present invention addresses these problems and provides in illustrative embodiments helically coiled tubes or hollow torsion bars of shape memory alloy which can be used in various industrial applications and which are capable of being heated in a variety of ways heretofore not employed in the shape memory alloy field.